I added these lines in between the default last </keybind> and </keyboard> in my ~/.config/openbox/rc.xml file (the line numbers are from vim). However, neither restarting X nor issuing an openbox --reconfigure applies these settings.

What am I doing wrong? I'm using Arch with all the default LXDE packages.

i've been struggling with the same problem for an hour now and the answer i'm afraid is quite silly and straightforward.
You should locate you keybinds within the keyboard block inside the rc.xml other wise they won't work.